METHYL PROPIONATE RADICAL-CATION

Examination of the reactions of the long-lived (> 0.5-s) radical catio ns of CD3CH2COOCH3 and CH3CH2COOCD3 indicates that the long-lived, non decomposing methyl propionate radical cation CH3CH2C(O)OCH3+. isomeriz es to its enol form CH3CH = C(OH)OCH3+. (Delta H-isomerization similar or equal to -32 kcal/mol) via two different pathways in the gas phase in a Fourier-transform ion cyclotron resonance mass spectrometer. A 1 ,4-shift of a beta-hydrogen of the acid moiety to the carbonyl oxygen yields the distonic ion CH2CH2C+(OH)OCH3 that then rearranges to CH3CH = C(OH)OCH3+., probably by consecutive 1,5- and 1,4-hydrogen shifts. This process is in competition with a 1,4-hydrogen transfer from the a lcohol moiety to form another distonic ion, CH3CH2C+(OH)OCH2. that can undergo a 1,4-hydrogen shift to form CH3CH = C(OH)OCH3+.. Ab initio m olecular orbital calculations carried out at the UMP2/6-31G* + ZPVE l evel of theory show that the two distonic ions lie more than 16 kcal/m ol lower in energy than CH3CH2C(O)OCH3+.. Hence, the first step of bot h rearrangement processes has a great driving force. The 1,4-hydrogen shift that involves the acid moiety is 3 kcal/mol more exothermic (Del ta H-isomerization = -16 kcal/mol) and is associated with a 4-kcal/mol lower barrier (10 kcal/mol) than the shift that involves the alcohol moiety. Indeed, experimental findings suggest that the hydrogen shift from the acid moiety is likely to be the favored channel.